1. Field of the Invention
This invention relates to the gasification of coal and similar carbonaceous materials and is particularly concerned with a catalytic gasification process carried out in the presence of a carbon-alkali metal catalyst to produce a chemical synthesis gas.
2. Description of the Prior Art
Existing and proposed processes for the manufacture of synthetic gaseous fuels from coal or similar carbonaceous materials normally require the reaction of carbon with steam, alone or in combination with oxygen, at temperatures between about 1200.degree. F. and about 2500.degree. F. to produce a gas which may contain some methane but consists primarily of hydrogen and carbon monoxide. This gas can be used directly as a synthesis gas or a fuel gas with little added processing or can be reacted with additional steam to increase the hydrogen-to-carbon monoxide ratio and then fed to a catalytic methanation unit for reaction with carbon monoxide and hydrogen to produce methane. It has been shown that processes of this type can be improved by carrying out the initial gasification step in the presence of a catalyst containing an alkali metal constituent. The alkali metal constituent accelerates the steam-carbon gasification reaction and thus permits the generation of synthesis gas at somewhat lower temperatures than would otherwide be required. Processes of this type are costly because of the large quantities of heat that must be supplied to sustain the highly endothermic steam-carbon reaction. One method of supplying this heat is to inject oxygen directly into the gasifier and burn a portion of the carbon in the feed material being gasified. This method is highly expensive in that it requires the existence of a plant to manufacture the oxygen. Other methods for supplying the heat have been suggested, but these, like that of injecting oxygen, are expensive.
It has been recently found that difficulties associated with processes of the type described above, can largely be avoided by carrying out the reaction of steam with carbon in the presence of a carbon-alkali metal catalyst and substantially equilibrium quantities of added hydrogen and carbon monoxide. Laboratory work and pilot plant tests have shown that catalysts produced by the reaction of carbon and alkali metal compounds such as potassium carbonate to form carbon-alkali metal compounds or complexes will, under the proper reaction conditions, equilibrate the gas phase reactions occurring during gasification to produce methane and at the same time supply substantial amounts of exothermic heat within the gasifier. This additional exothermic heat of reaction essentially balances the overall endothermicity of the reactions involving solid carbon and thus results in a substantially thermoneutral process in which the injection of large amounts of oxygen or the use of other expensive methods of supplying heat are eliminated.
The catalytic effect of carbon-alkali metal catalysts on the gas phase reactions, as distinguished from the solid-gas reactions or the reactions of carbon with steam, hydrogen or carbon dioxide, allows the following exothermic reactions to contribute substantially to the presence of methane in the effluent gas and drastically reduces the endothermicity of the overall reaction:
(1) 2CO+2H.sub.2 .fwdarw.CO.sub.2 +CH.sub.4 (exothermic) PA0 (2) CO+3H.sub.2 .fwdarw.H.sub.2 O+CH.sub.4 (exothermic) PA0 (3) CO.sub.2 +4H.sub.2 .fwdarw.2H.sub.2 O+CH.sub.4 (exothermic)
Under the proper operating conditions, these reactions can can be made to take place within the gasification zone and supply large amounts of methane and additional exothermic heat which would otherwise have to be supplied by the injection of oxygen or other means. Laboratory and pilot plant tests have shown that constituents of the raw product gas thus produced are present in equilibrium concentrations at reaction conditions and consist primarily of hydrogen, carbon monoxide, carbon dioxide, methane and steam. It has been proposed to utilize steam gasification in the presence of a carbon-alkali metal catalyst to produce a high Btu product gas by treating the raw product gas for removal of steam and acid gases, principally carbon dioxide and hydrogen sulfide; cryogenically separating carbon monoxide and hydrogen in amounts equivalent to their equilibrium concentration in the raw product gas from the methane in the treated gas; withdrawing methane as a high Btu product gas; and recycling the carbon monoxide and hydrogen to the gasifier. The presence in the gasifier of the carbon-alkali metal catalyst and equilibrium quantities of recycle carbon monoxide and hydrogen, which tend to supress reactions that would otherwise produce additional hydrogen and carbon monoxide, results in a substantially thermoneutral reaction to produce essentially methane and carbon dioxide. Since the overall reaction is substantially thermoneutral, only a small heat input is required to preheat the carbonaceous feed material and to maintain the reactants at reaction temperatures by compensating for heat losses from the gasifier. This small amount of heat may be supplied by preheating the gaseous reactants in a conventional preheat furnace.
It has also been proposed to utilize steam gasification of a carbonaceous feed material in the presence of a carbon-alkali metal catalyst to produce an intermediate Btu product gas by treating the raw product gas withdrawn from the gasifier for the removal of steam and acid gases, principally carbon dioxide and hydrogen sulfide; recovering a portion of the treated gas as the intermediate Btu product gas; contacting the remainder of the treated gas with steam in a steam reformer under conditions such that the methane in the treated gas reacts with the steam to produce additional hydrogen and carbon monoxide; and passing the effluent from the reformer into the gasifier. The amounts of hydrogen and carbon monoxide produced in the reformer compensate for the amounts of those gases removed in the treated gas that is withdrawn as intermediate Btu product gas. Thus the reformer effluent will normally contain carbon monoxide and hydrogen in amounts equivalent to the equilibrium quantities of those gases present in the raw product gas and will therefore supply the substantially equilibrium quantities of hydrogen and carbon monoxide required in the gasifier along with the carbon-alkali metal catalyst and steam to produce the thermoneutral reaction that results in the formation of essentially methane and carbon dioxide.